Pulse width measuring system



Nov. 26, 1946. E. LABIN ET AL 2,411,547

PULSE WIDTH MEASURING SYSTEM Filed March 26, 1943 col/puns T 26 J0 51?INVENTORS [IV/LE LflH/N 00/5641!) 0. GP/EG A 7720mm) Patented Nov. 26,1946 @FHQE PULSE WIDTH MEASURING SYSTEM Emile Labin, New York, andDonald D. Grieg, Forest Hills, N. Y., assignors to Federal Tole phoneand Radio Corporation, Newark, N. 3., a

corporation of Delaware Application March 26, 1943, Serial No. 480,624

13 Claims. 1

This invention relates to radio impulse systems and more particularly tothe measurement of the duration or width of pulses.

One of the objects of this invention is to provide a relatively simplemethod and means for measuring the widtlm of substantiallyrectangularlyshaped pulses.

Another object of the invention is to provide a pulse width measuringsystem requiring only a few parts and by which the width of pulse shapesor substantially rectangular form can be readily determined by a simpleoperation. I

In accordance with our invention, a pulse meas-' urement is made byapplying a train of the pulses to a shock excitable resonant circuitwhereby the leading and trailing edges of each pulse shock excite thecircuit to produce oscillations. The oscillations produced by the edgesof each pulse comblue to form an undulation following the trailing edgeof the pulse. The oscillations which normally follow this undulation aredamped out by providin a damping shunt connection across the resonantcircuit. This shunt connection includes a vacuum tube which when madeconductive provides a low resistance path across the resonant circuitthereby lowering the Q of the circuit to such a value as to absorbquickly the oscillat- -ing energy. The grid of the tube is arranged toreceive a negative voltage for the duration of each pulse while thepulse is applied to the resonant circuit. The anode and cathode of thetube are so connected that when the polarity of the current in theresonant circuit is in one direction it blocks conduction by the tubeand when in the opposite direction it unblocks the tube. By this arrangement, the tube operates to suppress the oscillations which wouldnormally follow the desired undulation by the tubebecoming conductivewhen the voltage across the resonant circuit changes in no larity at theend of the undulation and when the voltage of th applied pulse becomeszero.

The system according to our invention will, therefore, produce anundulatlon following the trailing edge of each pulse and thereafter suppress all further oscillations until the next suc ceeding pulse isapplied to the circuit.

By providing the resonant circuit with an amplifier stage and amaximizing-and minimizing in dicating meter or other maximizing orminimiz ing indicator, such as a cathode ray oscillograph,

an operator can easily determine when the circuit is tuned to thefractional period corresponding to the duration of the pulse. That is tosay, the leading and trailing edges of the pulse are then in eitherdirect step or opposition with the in duced oscillations of the circuit.The tuning control of the circuit may be calibrated to give a directreading of the pulse width when the greatest maximum or minimumindication for the undulation is obtained.

For a further understanding of the invention, reference may be had tothe following detailed description to be read in connection with theaccompanying drawing, in which:

Fig. l is a schematic wiring and block diagram of the pulse widthmeasuring system according to our invention; and

Fig. 2 is a graphical illustration of the steps car ried out by thesystem during a tunin operation.

Referring to Fig. 1, the pulse width measuring circuit is shown providedwith an input connection l0 havin a coupling stage i2 which ispreferably a screen grid stage, whereby positive pulses i5 are invertedas indicated at 15a for application to the resonant circuit it. Inaddition to inverting the pulses this coupling stage presents a highresistance with respect to the applied voltage so that the total currentflow into the following circuit is an exact replica of the appliedpulse. Com necting the output I! of the coupling stage iii to theresonant circuit i8 is a resistor R. The resonant circuit includes anadjustable condenser C and an inductance coil .L. The condenser and thecoil are connected in parallel. Connected across the terminals l8 and 19of the inductance coil L is a damping tube 20. The output connection iiis connected by lead 22 to a grid 23 of the tube. The cathode it isconnected to the terminal i8 and the anode 25 is connected to theterminal i9. The terminal i9 is also connected to a source of positivepotential 13+. Condenser 01 represents a low return impedance to groundand is not part of the resonant circuit.

The potential or" the resonant circuit present at the terminal i8 istalsen ofl at an output 28 forapplication to an amplifier stage 30 whichis pref erably arranged to operate as a, class "C" amplifier. The anodeconnection 32 oi. the amplifier 30 is connected to a meter 31 theopposite side or which is connected to a positive potential 13+.

The meter 31 may be of any suitable character long as it gives anindication of peak voltage of the undulations passed by the amplifier 30to the anode circuit 32.

The operation of the system of 1 will be better understood by referenceto Fig. 2 in which all curves are applied to the same time base. Curve arepresents the pulse input after passing the coupling stage. Thus, thepulses l5 are negative this point in the circuit. Should the inputpulses 3 be negative instead of positive the coupling stage l2 could bereplaced with an input resistance. For purposes of illustration, thepulses i of curve a may also be regarded as the voltage across theresistor R since the total current that flows into the tuned circuitwill be anexact replica of the applied input voltage.

aerate? Curve b represents the oscillations at the output cuit it whenthe circuit is tuned to a wavelength it which is greater than twice theduration of the pulse iii. Curve 2 represents the output potential ofthe circuit id when the circuit is tuned to a wavelength it which isin'the order of an odd multiple or the duration of the pulse.

Referring particularly to the curves (1 and b, the leading edge ii ofthe puls will initiate an oscillation lii. plied to the circuit it, itinitiates a second oscillation dd. Since the tuning of the circuit id isout of step with the width of the pulse 15, the oscillations 62 and d3are out of step. These oscillations combine algebraically to produce anundulation 6S.

Assuming that the damping tube 29 is removed from the circuit of Fig. 1,the oscillations 32 and 44 will continue to combine beyond theundulation 55 thereby producing a damped oscillation 4i shown in brokenlines in curve I). For a resonant circuit of relatively high Q" theoscillation 48 would be present when the next succeeding pulse 15?) isapplied to the circuit. It will be apparent from the curve b that thisoscillation QB in such case will combine with the oscillations.

produced by the pulse i5b thereby affecting the shape of the undulationit? which would be normally produced by the pulse I52 As hereinbeforeexplained the damping tube 2b is provided to damp out the oscillationssuch as 56 which would normally follow undulations 45, it, etc, so thatthe oscillations produced by one pulse will not affect the oscillationproduced by the next succeeding pulse. This damping out operation isperformed when the tube 2b is rendered conductive as when the undulation6-5 reaches the zero potential oi the curve b. Thus, when the energy ofthe circuit 56 starts to reverse in polarity at the terminals i8 and it,the tube is rendered conductive thereby reducing the "Q of the circuitIt to such a value as to absorb the oscillatory energy of the circuit.This absorption of the oscillatory energy is such as to substantiallycompletely damp out the oscillations that would normally follow theundulation 65. This results in a zero potential line 5Q following theundulation 45 until the next succeeding pulse is applied to the circuit.

The conduction of the tube Ed is prevented during the application of apulse to the circuit is by the connection 22 whereby the negative energyof the pulse (curve a) is applied to the grid 23 of tube. This insuresthe blocking of the tube against conduction for the duration of thepulse, and after the trailing edge of the pulse, the undulation it beingpositive at the terminal l8 prevents the tube from conduction for theduration of the undulation.

By adjusting the condenser C, the undulation 35 may be minimized ormaximized. The adjust- When the trailing edge 53 is apment for thecondition of curve 23 will give a meter indication M1 while the tuningadjustment represented by curves 0, d and e will give meter indi.catlons Ma, Ma and M4, respectively. These indications are representedon curve 0 whereby a comparison may be easily seen. It will be clearthat the undulation indication 55c of curve 0 represents the maximumobtainable by adjustment of the condenser C. Thus, the period 22 whichrepresents adjustment of the condenser C for the condition illustratedin curve 0 is the period which is substantially twice the duration ofthe pulse 45. The calibration of the condenser C, of course, will besuch as to give a direct indi= cation in microseconds of the width ofthe pulse maximized, this indication being dependent only on the passiveelements C and L.

,For increased measurement range, additional inductance coils may beprovided together with switching means for substituting selectively acoil of desired inductance value for the inductance coil L. In thatcase, the condenser C will be provided with a separate calibration foreach coll. A tuning arrangement of this character is disclosed in thecopending application of Donald D. Grieg entitled Measuring system,Serial-No. 475,734, filed February 13, 1943.

In the case of the circuit being tuned so that an odd multiple ofhalf-periods occur during the pulse width, an example is indicated bycurve 2,

wherein the wavelength is 1 /2 times the pulse a width. It will be notedthat the maximized undulation a is less than the maximum undula-- tionindication 355c (curve 0) The reason for this difference will be readilyapparent from curve e because of the damped condition of the oscillation52c produced by the leading edge iii. The second positive undulation d2of the oscillation die is considerably less than the first positiveundulatlon 58. Since it is the second undulation 52 which is in stepwith the undulation tide produced by the trailing edge 53, theundulation lite resulting from the combining action of the undulation'ite and 52 is less than the maximized indication ldc. Thus, while therepresents a maximized indication for a wavelength in the order of anodd multiple of the pulse width, it is clear that such maximizingoperation does not produce the reatest maximum indication possible.

In general a maxima is obtained when the period of the tuned circuit issuch that:

l Z=;T

where w is the width of the pulse and n1 is an odd integer correspondingto the number of half periods occurring during the pulse time.

A minima is obtained when:

where m is an even integer corresponding to the number of half periodswithin the pulse time.

In order to prevent the undulation 5! from being'applied to the meter atfor the tuning condition illustrated in curve e, the negative potentialof the pulse 55 (curve a) is applied to the amplifier stage 36 through aconnection 55. This negative potential for the duration of the pulse i5is used to block out the amplifier in known manner so that for theduration of the pulse any positive undulations produced will not beapplied to the meter 3 This blocking of undulation 5| is indicated atdia.

While we have shown and described the prinaai 1,54"?

ciples of our invention in connection with specific apparatus, werecognize that various changes and modifications may be made thereinwithout departing from the invention. For example, instead ofmaximizing, the system may be arranged to give pulse width measurementsby a minimizing operation. It is our aim therefore to cover in theappended claims all such changes and modifications as fall within thescope of the invention.

We claim:

1. A method of measuring the duration of pulses of a given shape havingabrupt changes at the leading and trailing edges thereof, comprisingshock exciting a resonant circuit by the leading and trailing edges of atrain of such pulses, the oscillations initiated by the edges of eachpulse combining to form an undulation following the trailing edge of thepulse, damping out the oscillations which normally follow saidundulation so that the oscillations produced by one pulse will notafiect the oscillations produced by the next succeeding pulse, andtuning the circuit until the undulation reaches a size limit such as itsgreatest maximum, whereby one-half the period-of the frequency to whichthe circuit is tuned represents a timing at a given multiple of theduration of the pulse.

2. The method defined in claim 1, wherein the damping out ofoscillations is controlled by a given polarity condition across thetuned circuit after occurrence of each pulse.

3. The method defined in claim 1, wherein the damping out of theoscillations is produced by blocking the energy of the oscillations whenthe energy changes polarity at the end of said undulation.

4. A system for measuring the duration of a pulse comprising a shockexcitable circuit, means to apply a train of the pulses to the circuitto produce separate oscillations therein in response to the leading andtrailing edges of each or" the pulses, the oscillations initiated by theleading and trailing edges of each pulse combining to form. anundulation following the trailing edge or" the pulse, means to damp outthe oscillations which normally follow said undulation so that theoscillations of one pulse will not affect oscil lations produced by thenext succeeding pulse, means to indicate energy of said undulation, andmeans to tune the circuit until said undulation reaches a size limitsuch as its greatest maximum, whereby one-half the period of thefrequency at which the circuit is tuned represents a given multiple ofthe duration of the pulse.

5. The system defined in claim 4, wherein the means to damp out theoscillations comprises a low resistance shunt connection across saidcircuit and means responsive to energy 01! said pulse to control theblocking and unblocking of said shunt connection.

6. The system defined in claim 4, wherein the means to damp out theoscillations comprises a low resistance shunt connection across said circuit and means responsive to energy of said pulse and oscillatory energyfrom said circuit of polarity opposite the polarity of said undulationto block the shunt connection.

7. The system defined in claim i, wherein the means to damp out theoscillations comprises a t3 vacuum tube connected across said circuit,and means to bias the tube to cut-ofi for the duration of the pulse, thepolarity of the circuit con-- nections across the tube being such duringthe 5 formation of said undulation as to maintain the tube at cut-offfor the duration of said undulation and the change in polarity of saidconnections at the end of said undulation operating toproduce conductionin said tube to shunt said circuit.

8. The system defined in claim. l, wherein the means for indicatingenergy of said undulation includes an amplifier arranged to thresholdclip the undulations.

9. The system defined in claim l, wherein the means for indicatingenergy of said undulation includes an amplifier arranged to thresholdclip the undulations and means connected to the input of the shockexcitable circuit to apply the energy of the pulses to the amplifier toblock operation thereof for the duration of each or" the pulses so thatundulations produced by the leading edge prior the occurrence of thetrailing edge of a pulse will not be passed by said amplifier.

10. A system for translating the pulses of a train of like pulses intoundulations comprising a shock excitable circuit, means to apply thetrain of pulses to the circuit to produce oscillations therein inresponse to the leading and trailing edges of each of the pulses, theoscillations initiated by the leading edge of a pulse combining to forman undulation following the trailing edge of the pulse, a vacuum tubehaving anode, cathode and grid electrodes, means connecting the anodeand cathode electrodes across said circuit, and means to apply saidtrain of pulses to said grid electrode to block said tube for theduration of each pulse, the anode and cathode connections being arrangedto produce a blocking potential for the tube for the duration of saidundulation and an unblocking potential for said tube upon initiation ofa voltage having polarity opposite to the polarity of said undulation,whereby said tube clamps out oscillatory energy in said circuitfollowing said undulation.

11. A circuit comprising a plurality of circuit elements connected inparallel circuit arrangement, one of said elements being a condenser, a

50 second being an inductance and a third being a damping tube havingthe cathode and anode electrodes thereof connected in the parallelcircuit,

a resistor, an input for negative pulses connected through said resistorto said parallel circuit, said 55 tube having a grid, and meansconnecting said input to said grid.

12. The circuit defined in claim 11, in combi nation with an energyindicator including an am plifler, and means connecting the parallelcircuit to to the input of said amplifier.

13. The circuit defined in claim ii in combine. tion with an energyindicator including an arm plifier, means connecting the parallelcircuit to the input of said amplifier, and means to connect 65 theinput to said parallel circuit to said amplifier to maintain theamplifier block during application of a negative pulse to the input ofsaid circuit.

